It is preferable to acquire all products for Pesach, prior to Pesach.
The ruling of the Mishneh Berurah is unequivocal - Ashkenazi custom bans consuming Kitniyot, rice etc. during Pesach, but only if they are discernible.
Ashkenazim may consume foods containing Kitniyot when A) Kitniyot is less than 50% of the product B) Kitniyot are not visually identifiable C) these foods were not specifically made for those who keep the customary ban of not eating Kitniyot. ...... Therefore lemonades etc. which may use corn syrup, and chocolates which may use lecithin are Kosher for Pesach.
Liquid or creme fillings in chocolates might well contain Chamets and require KLP certification.
Soft drinks etc. may be consumed during Pesach even if the carbon dioxide gas is produced from Chamets or Kitniyos.